prost = [2]
n = 10000
for i in range(3,n+1):
    ksun = 0
    for j in prost:
        if i %j == 0:
            ksun=1
    if ksun == 0:
        prost.append(i)

a = input ()
b =[]
if int(a) in prost:
    b.append(a)
b.extend([a[:-1],a[1:],a[:-2],a[2:],a[1:-1],a[0],a[1],a[2],a[3]])

x = []
for i in range(len(b)):
         s=b[i]
         for j in range(len(prost)):
             if int(s) == int(prost[j]):
                 x.append(int(s))
if len(x) >0:
    print(max(x))
else:
    print(0)
        
